Generally, when manufacturing a semiconductor device, various processes such as a film forming process and a pattern etching process are repeatedly performed on a semiconductor wafer to obtain a desired device. Especially, in order to fill a recess on the semiconductor wafer or in order to form a wiring pattern on the semiconductor wafer, a thin metal film made of, e.g., Ti, Ta, Cu or Al may be formed. In this case, in order to prevent a thermal damage to an underlayer already formed on the semiconductor wafer in a previous process, there has been widely used a plasma sputtering method capable of forming a metal film having relatively good characteristics at a low temperature.
For example, Japanese Patent Laid-open Publication No. 2006-148075 describes a film forming apparatus using a plasma sputtering method. FIG. 5 is a schematic configuration view showing an example of a film forming apparatus using a plasma sputtering method. As shown in FIG. 5, a metal target 4 as a source material for a metal film is provided at a side portion of a ceiling within an evacuable processing chamber 2. A mounting table structure 6 for mounting thereon a semiconductor wafer W is provided at a central portion within the processing chamber 2. The mounting table structure 6 includes a mounting table 9 having thereon an electrostatic chuck 8 configured to attract and hold the semiconductor wafer W by applying a high DC voltage. The electrostatic chuck 8 also serves as an electrode 10. A high frequency bias power supply 12 is connected to the electrode 10 in order to attract metal ions ionized by plasma. A transmission plate 14 is provided at the ceiling of the processing chamber 2. The transmission plate 14 is made of, for example, quartz, and a high frequency power passes through the transmission plate 14. An induction coil 18 is provided outside the processing chamber 2. The induction coil 18 is connected to a high frequency power supply 16.
A high frequency power is applied from the induction coil 18 into the processing chamber 2 through the transmission plate 14. A gas such as an Ar gas introduced into the processing chamber 2 is excited into plasma P. Metal particles are ejected from the metal target 4 by colliding the plasma P with the metal target 4. The metal particles are increasingly ionized by the plasma P and attracted to the mounting table 9. Accordingly, a metal film is deposited on the semiconductor wafer W.
In this case, the metal film is also deposited on an inner wall surface of the processing chamber 2 or on surfaces of components within the processing chamber 2 as well as on a surface of the semiconductor wafer W. In order to prevent such unnecessary deposition of the metal film, a protective cover member 20 is provided so as to substantially surround a side surface of the mounting table 9 and a processing space where the plasma P is generated. An insulation gap 22 having a small width is formed between the protective cover member 20 and the side surface of the mounting table 9. The protective cover member 20 is made of a conductive material such as SUS or aluminum, and is connected to a ground side. An unnecessary metal film is deposited on a surface of the protective cover member 20.
However, in the above-described film forming apparatus using the plasma sputtering method, the metal film is deposited not only on the surface of the semiconductor wafer W but also on a periphery of a top surface of the mounting table 9 that is not covered by the semiconductor wafer W and a top surface of the protective cover member 20 surrounding the mounting table 9. As a result, an unnecessary metal film 24 is formed.
If a thickness of the unnecessary metal film 24 is thin, no particular problem may occur. However, as the thickness of the unnecessary metal film 24 is gradually increased, the width of the insulation gap 22 that is set to be, for example, about 1.5 mm becomes substantially narrower. In that case, an electrostatic capacitance of a stray capacitance 26 (refer to FIG. 6) in the insulation gap 22 is significantly increased. Accordingly, a high frequency bias power leaked to the ground side through the stray capacitance is gradually increased, so that a high frequency power applied to the plasma is changed. As a result, uniformity in a film forming process on the semiconductor wafer W is deteriorated, so that a plasma process becomes non-uniform as time passes by.
To be more specific, FIG. 6 is an equivalent circuit showing a high frequency bias power supply side of the plasma film forming apparatus shown in FIG. 5. Here, the plasma P is presented by a parallel circuit of a capacitor C and a resistor R. As described above, since the stray capacitance 26 is generated between the protective cover member 20 (ground side) and the electrode 10(8) (hot side) to which a high frequency power is applied, the electrostatic capacitance of the stray capacitance 26 is changed due to the deposition of the unnecessary metal film. Accordingly, the high frequency bias power leaked to the ground side via the stray capacitance 26 is increased, so that the high frequency bias power applied to the plasma P varies. As a result, uniformity in the plasma process on the semiconductor wafer W is deteriorated, so that the plasma process becomes non-uniform as time passes by.